Hafnium-containing columbium-base alloys



United States Patent 3,341,37 0 HAFNIUM-CONTAINING COLUMBIUM-BASE This invention relates to new and improved columbium base alloys that contain hafnium and a solid solution strengthener from the group consisting of tungsten, molybdenum and mixtures thereof; it also relates to a meth- 0d of making such alloys. More particularly, this invention relates to a method for improving the high-temperature stress-rupture strength of such alloys without sacrificing either their low-temperature ductility properties or their high-temperature structural stability, which method thereby imparts to these alloys a utility that they would otherwise be denied.

The principal limitation in gas turbine technology today is the maximum turbine inlet temperature. The turbine inlet temperature is, in turn, set by the temperature that the turbine vanes and blades are able to withstand without danger of failure. Formerly, the best available high temperature alloys were nickel and cobalt base super-alloys, but critical structural components, such as turbine vanes and blades constructed from such alloys, are limited to maximum operating temperature of between about l600 and 1900 F.

For many years it has been generally known that the high temperature strengths of metals are closely related to their melting points. Thus, metals having high melting points also tend to have high temperature strength potentials.

The need for structural materials for service at temperatures in excess of those obtainable with existing ma: terials of construction, such as, nickel and cobalt alloys, has stimulated interest in the metals having the highest melting points, or the refractory metals, particularly chromium, columbium, molybdenum, and tungsten. Until recently molybdenum was considered the chief prospect for such usage, however, at the high temperature service conditions needed, molybdenum oxidizes at a catastrophic rate, principally because molybdenum oxide is volatile at elevated temperatures.

As an alloy base material for high-temperature serv-' ice, columbium offers promise, and considerable interest has been directed to its use as a structural alloy base for applications in high-temperature environments. Among the technically most important physical qualities of columbium as an alloy base are its high melting temperature (4380 F.) and its low neutron-capture cross-section. Columbium is, therefore, potentially useful for such applications as fast aircraft, space flight vehicles, and nuclear reactors.

Further, columbium is inherently a soft, ductile, readily fabricable material. Although its melting temperature is about 4380 F., pure columbium becomes too Weak for practical structural uses at temperatures above 1200 F. Columbium is also a very reactive metal in that it dissolves large quantities of oxygen and probably nitrogen, on exposure to atmospheres containing even small amounts of these elements at modest temperatures.

Thehistory of columbium alloy technology has demonstrated the incompatability of achieving oxidation resistance and high-temperature strength through alloying alone. Since the major use for columbium base alloy is as structural components in high-temperature applications, it is apparent that useful classes of columbium alloys will demand protective coatings in their normal hightemperature oxidizing environments.

In contrast to molybdenum which oxidizes catastrophically, columbium oxide does not volatilize, and it is thus potentially possible to prevent oxygen attack on columbium by coating the metal, and if premature localized coating failure should occur, to restrict the failure and oxygen attack to the localized site. Further advantages offered by columbium over molybdenum base alloys are that columbium base alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum.

To achieve desirable strength and stress-rupture properties in columbium base alloys for service as structural components in gas-turbine engines for operation of 2200 'F., columbium can be most effectively strengthened by a solid-solution strengthener selected from the group consisting of tungsten, molybdenum, and mixtures thereof. Auxiliary additions of one or more elements selected from the group consisting of tantalum, zirconium, vanadium, and beryllium, may provide additional improvements in low-temperature ductility, density or high-temperature strength over the simpler binary or ternary alloys.

Both tungsten and molybdenum additions, which must be regarded as the primary strengtheners, have the disadvantage of increasing the ductile-t o-brittle transition temperature, and thus degrade the low-temperature ductility that is characteristic of nnalloyed columbium. Although this undesirable effect can be counteracted by tantalum additions or by controlled fabrication techniques to an extent sufiicient to impart measurable room-temperature ductility to the strengthened alloy without loss of struc tural stability, there still exists an alloying limit beyond which the addition of tungsten and/or molybdenum will result in no useful ductility at a given low reference temperature. v

Copending application Ser. No. 65,962, filed Oct. 31, 1960, now abandoned, discloses and claims a class of fabricable, ductile, stress-rupture resistant columbium base alloys that will readily fulfill the structural requirements for use at high temperatures up to at least 2500 F. Typical of this latterclass of alloys is the composition Cb20Tal5W-5Mo (additions expressed in percent by weight).

Although auxiliary-strengthening additions of hafnium to columbium-base alloys containing tungsten or molybdenum, or both of these, either with or without auxiliary additions of tantalum or vanadium, or both of these, previously have been realized to be advantageous, this invention relates to a method whereby the effectiveness of strengthening zby auxiliary hafnium additions can be significantly and surprisingly increased to yield an improved alloy possessing importantly increased high-temperature stress-rupture strength.

It is, accordingly, a primary object of this invention to provide a method of treating solid-solution-strengthened columbium base alloys that contain auxiliary-strengthening additions of hafnium in a manner whereby the alloys achieve a significant increase in high-temperature stressrupture strength without sacrificing the desirable room temperature ductility properties of the alloy base and without sacrificing the structural stability of the alloy during subsequent high-temperature fabrication or service.

7 It is another object of this invention to provide a process for making new and improved hafnium-containing columbium base alloys having a high-temperature stress-rupture strength superior to any columbium base alloys heretofore known for which useful room-temperature ductility has been demonstrated, which process introduces light retained work-hardening into the alloys and achieves this without a concomitant degrading of their low-temperature ductility properties and without adversely affecting their structural stability and resistance to recrystallization dur ing high-temperature fabrication or service.

A further object of this invention is to provide new and improved columbium base alloys and a process for making them that achieves alloys having superior stressrupture strength at temperatures up to at least about 2500 F. without sacrificing their ductility at room-temperature and without sacrificing their structural stability during high-temperature fabrication or service conditions.

Additional objects and advantages of the invention will be set forth in part in the description that follows and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the compositions, methods, and processes particularly pointed out in the appended claims.

It has been found that these and other objects of this invention can be realized by a method for improving the high-temperature stress-rupture strength of a columbium base alloy without sacrificing its low-temperature ductility properties and its structural stability during subsequent high-temperature fabrication or service, such columbium base alloy containing columbium as the primary ingredient and an additive in an amount sufiicient to significantly improve the high-temperature stress-rupture strength of elemental columbium, the additive consisting essentially of hafnium and a solid-solution-strengthener selected from the group consisting of tungsten, molybdenum and mixtures thereof; the method, broadly described, comprising the steps of heating the alloy to a temperature of from 2800" F. to 3200 F. and then subjecting the alloy in a specimen of a given cross-sectional area, while at such elevated temperature, to forces so as to effect plastic deformation of the alloy, whereby the cross-sectional area of the alloy is substantially reduced. By the foregoing method a condition defined as light retained work-hardening is induced in the metallurgical structure of the alloy.

columbium base alloys treated in the manner just described have been found to possess exceptional hightemperature stress-rupture strength, measurable roomtemperature ductility, and adequate structural stability at high temperatures.

In a particularly advantageous method of practicing this invention, the force to effect plastic deformation is of such a nature that the alloy is reduced in cross-sectional area by an amount of from 40 to 90 percent of its initial cross-sectional area, and in a preferred method of practicing this invention, the alloy specimen is reduced in cross-sectional area by an amount of from 60 to 70 percent of its initial cross-sectional area. Also, in a preferred method of practicing this invention, the alloy is heated to a temperature of from 2900 to 3100 F. before the forces to effect plastic deformation of the alloy are introduced.

The alloy or product obtained from the method of this invention may be described as a columbium base alloy having an excellent high-temperature stress-rupture strength, room-temperature ductility, and structural stability during fabrication or service at high temperatures. The alloy consists essentially of columbium as the primary ingredient and an additive in an amount sufficient to improve the high-temperature stress-rupture strength of elemental columbium; the additive consists essentially of hafnium in an amount of up to 10 percent by weight and a solid-solution-strengthener selected from the group consisting of tungsten, molybdenum, and mixtures thereof, the solid-solution-strengthener being present in an amount of from 5 to 30 percent by weight; the alloy is characterized by having a light retained work-hardened structure imparted by reducing the cross-sectional area of the alloy in a previously strain-free specimen by from l 40 to percent while the alloy is at a temperature of from 2800" to 3200 F.

When tungsten and molybdenum are added to columbium as solid-solution strengtheners, in accordance with this invention, tungsten is generally added in an amount from 5 to 30 percent by weight, and preferably from 15 to 25 percent by weight, while molybdenum is generally added in an amount of 5 to 25 percent by weight, and preferably from 10 to 20 percent by weight to achieve significantly improved stress-rupture strength at temperatures up to at least 2500 F. Of course, when both tungsten and molybdenum are incorporated for this purpose, the amount of each employed may be proportionately lower than that indicated above. In a particularly advantageous form of the alloys, when both tungsten and molybdenum. are used, tungsten may be present in an amount of from 10 to 20 percent by weight and molybdenum in an amount of from 2 to 10 percent by weight, and in a preferred form of the alloys the ratio of tungsten to molybdenum in percentages by weight is three to one (3 to 1). Then, too, the columbium base alloys treated in accordance with this invention may contain amounts of other metals that may be normally added to columbium base alloys for improving the properties thereof, such as, for example, tantalum, in an amount of up to 40 percent by weight and preferably from 15 to 40 percent by weight, and the alloys may also contain, either with or without tantalum, one or more elements selected from the group consisting of zirconium, vanadium, and beryllium, in amounts of from 0.2 to 5 percent by weight each, but the total of such added elements in the alloys should not be more than 10 percent by weight.

Thus the overall alloy compositions that can be worked in accordance with the method of this invention to produce the improved alloy products derived from that method can be concisely defined as consisting essentially, by weight, of

Percent Hafnium up to 10 A solid solution strengthener selected from the group consisting of tungsten, molybdenum and mixtures thereof 5 to 30 Tantalum 0' to 40 Zirconium 0 to 5 Vanadium 0 to 5 Beryllium 0 to 5 Essentially columbium balance provided that the alloy contains not more than 10% by weight of zirconium, vanadium, and beryllium, in aggregate.

Hafnium, which is an essential ingredient and of critical importance to the new and useful result of the present invention, may be present in an amount of up to 10 percent by weight of the alloy, is advantageously present in amounts from 0.5 to 5 percent by weight, and in the most preferred form of the invention is present in an amount of from 2 to 3 percent by weight of the alloys.

The relationship existing between stress-rupture strength and the amount of primary solid-solution-strengthening additions (tungsten and molybdenum) to columbium in forming particularly desirable structural alloys may be expressed empirically as follows:

where a is the stress required for rupture time t, A and B are constants for a given rupture time t, and [C] is the effective atomic concentration of primary strengtheners (W-l-Mo), equal to atomic percent W-|-0.85 atomic percent Mo.

It has been reliably estimated that for rupture in hours at 2200 F., the constants A and B assume values of 2.5K p.s.i. and 1.29K p.s.i./atom percent, respectively, and the relationship becomes:

0' 1000 p.s.i.)=2.5+ 1.29 [C] F. By way of illustration, as determined l5W-5Mo alloy (expected l-hour recrystallization-initia- Experimental determination of rupture strengths of alloys containing up to at least 22 atomic percent combined additions of tungsten and molybdenum agreed with this parameter usually to within about percent in the absence of structural instability (recrystallization) during testing.

It has been generally recognized that for structural service at elevated temperatures, refractory metals and alloys, including columbium alloys, can effectively utilize workhardening as a strengthening mechanism. It is important, however, that alloys which utilize retained work-hardening as a high-temperature strengthening mechanism also possess adequate structural stability or resistance to recrystallization during the stresses, temperatures, and times that constitute the service conditions for the alloys in question.

In this regard, with reference specifically to columbium alloys, the amount and type of alloying additions, and the degree of retained cold work are particularly significant. Both tungsten and molybdenum additions to columbium are effective in increasing the temperature required for recrystallization. It has been found, for example, that in a moderately-worked condition, the temperature required to initiate recrystallization in 1 hour is increased by about 50 F. for each atomic percent addition of tungsten, or for approximately each 1%. atomic percent addition of molybdenum in total concentrations up to about at least 22 atomic percent. Thus, in the following alloys, satisfactory structural stability at a temperature of 2200 F. would be expected to persist for appreciable times.

Temperature for Initiation of Recrystallization Alloy, Percent by Additions, Atomic in 1 hr., F.

Weight Percent Calculated Observed CbW'7.5M0 Cb5.33)V-7.65Mo 2, 370 2, 350 Cb-20W10Mo Cb-ll .27Vv10.78M0 2, S00 2, 800

As a crude approximation, it is estimated that the logarithm of time is inversely related to a linear function of temperature in describing the kinetics of the recrystallization process, such that a decrease of 150 F. requires about a l0-fold increase in the time required for the same degree of recrystallization as at the higher temperature or reference temperature.

Tantalum additions to columbium exert little or no effect on recrystallization temperature. Hafnium additions, which are not ideal solid-solution-strengthening additions by virtue of their tendency to scavenge interstitial elements (carbon, oxygen, nitrogen, and the like) to form dispersed foreign phases, may provide additional slight benefits to tungstenand/or molybdenum-containing alloys, when used in moderation (e.g., 5 percent by weight or less). But hafnium additions in columbium base alloys not treated in accordance with this invention have been observed to be markedly detrimental to recrystallization behavior at the level of 10 percent by Weight. Vanadium additions to structural alloys degrade their structural stability at all levels of addition, but because of proportionate desirable effects, such as improving resistance to scaling, they can be tolerated at modest levels (5 percent by weight or less).

More severe degrees of working (e.g., 90 percent reduction in cross-sectional area at 1800 F., or about 60 percent reduction at 500 F.) result in recrystallization temperatures that are lower by about 100 F. than those that result from moderate degrees of working (e.g., about 60 percent reduction in area at l800 F.), whereas light degrees of working (e.g., about 60 percent reduction in area at temperatures of 2900 to 3100 F.) result in recrystallization temperatures that are higher by about 100 for a Cb20Ta tion temperature of 2490 F. in the moderately-worked condition), temperatures required to initiate recrystallization, with prior histories that resulted in the different Whereas heavily-worked materials (referred to herein as conditions H or VH) are desirable to increase-the amount of strain-hardening (hence, both strength at high temperatures and ductility at low temperatures), the amount of work- (or strain-) hardening is limited by the stability of the worked structures under service conditions. The most stable worked structures (e.g., those only subjected to light work-hardening, referred to herein as condition L) may not retain enough strain-hardening during high-temperature fabrication to significantly improve their high-temperature strength and low-temperature ductility.

To aid in a clearer understanding of the invention, specific examples of the invention are herein set forth. These examples are merely illustrative, however, and should not be understood as in any way limiting the scope or underlying principles of the invention. The ensuing examples, illustrative of the invention, are presented to demonstrate the effects of the invention upon the properties of solidsolution-strengthened hafnium-containing columbium base alloys. More specifically, and in accordance with the invention, these examples delineate the methods of fabricating or treating such alloys to achieve markedly im proved strength from the auxiliary hafnium additions.

To facilitate an understanding of the following examples, the standard procedures and steps used in consolidating, fabricating and testing the alloys forming the examples of this invention are set forth below:

(l') C0ns0lidati0n.-Charges of the appropriate elements were melted on a cooled copper crucible under a /3 atmosphere of high-purity helium using electric arc melting with a tungsten electrode. Alloys were melted from 10 to 15 times to insure adequate homogenization, and were finally cast to button ingots measuring nominally 2% x 4 x inches.

(2) Fabricwti0n.Four different were used as follows:

(a) Arc-cast ingots were machined and encased in thin sheets of molybdenum and inserted in tight-fitting holes in steel yokes. Steel cover plates were welded in place, and the pack assemblies were evacuated to about 0.1 micron of mercury at 1800 F. and sealed under vacuum. The assemblies were then rolled at 1800 F. to produce alloy strips nominally 0.040-inch thick. This effected about a percent reduction in cross-sectional area and resulted in alloy strips containing about 90 percent retained warm work (where Warm work is defined as work induced at a temperature that is above ambient temperature but below the temperature at which recrystallization is initiated for the particular alloy, working conditions and times involved). The degree of retained workhardening was very heavy, and this condition is designated VH.

(b) Pack assemblies were prepared as in (a) above, but were rolled at 1800 F. to effect only about 75 percent reduction in the alloy strips. Alloy strips were recovered, cleaned and annealed to recrystallize the structures, thus removing any retained work-hardening effects and creating a strain-free structure. The alloy strips were then rolled at low temperatures (70 to 500 F.) to effect total reductions of from 60 to 70 percent By virtue of fabrication teachings the low final rolling temperature, heavy work hardening was retained, and this condition is designated H.

(c) Alloys were processed identically as in (b) above through the intermediate recrystallization annealing treatment. The recrystallized strips were reencapsulated in suitably designed steel packs (with molybdenum shims), hot-evacuated, and rolled to nominal 0.035-inch-thick strip at 1800" F. with a total final reduction of between 60 and 70 percent. Because of the intermediate final rolling temperature of 1800 F. and the amount of the reduction accomplished, the degree of retained workhardening was moderate, and this condition is designated M.

((1) Alloys were processed identically as in (b) above through the intermediate recrystallization annealing treatment. The recrystallized strips were encapsulated in suitably designed molybdenum yoke-cover plate assemblies, with closure being accomplished by welding under argon. These assemblies were rolled at temperatures of from 2900 to 3100 F. to final strip thickness of about 0.040 inch to effect 60 to 70 percent reduction in cross-sectional area. Because of the very high temperatures of fabrication, only a light" degree of retained work hardening resulted, and this condition is designated L.

(e) Subsequent to fabrication, under all four of the above fabrication teachings, the alloy strips were conditioned by grinding as necessary, and by annealing for /2 hour at 2200" F. to alleviate any undesirable residual stresses prior to mechanical testing. Standard sheet tensile and stress-rupture specimens were machined from these alloy strips.

(f) Although the foregoing /2 hour process annealing step was applied to all materials tested and although for more desirable combinations of alloy content and metallurgical structure this step would be a beneficial stressrelief treatment (i.e., would alleviate undesirable residual stresses), for other combinations of alloy composition and metallurgical structure, especially those with the least alloying additions and/r more severely worked structures, it is probable that some recrystallization was induced thereby impairing the material properties of such alloys or examples.

(3) Testing.

(a) Recrystallization temperatures for the test materials were determined by metallographic examination of specimens that had been vacuum annealed for 1 hour periods at selected temperatures. The temperature requirement for initiation of recrystallization is the most critical value in considering the alloys for time-dependent stress-rupture application. For most alloys of the examples, the temperature required for the completion of recrystallization in 1 hour was about 400 F. higher than that required to initiate recrystallization in 1 hour.

(b) Tensile tests at room temperature were conducted in hydraulically loaded testing machines at strain rates of 0.005 inch per inch per minute up to plastic strains slightly in excess of 0.2 percent, and 0.05 inch per inch per minute thereafter to fracture.

(c) Tensile tests at 2200" F. were conducted by progressive dead-weight loading to provide a strain rate of about 0.1 inch per inch per minute throughout the test. These tests were conducted in vacuum.

(d) Stress-rupture tests were conducted in vacuum of greater than 1 micron of mercury at 2200 F. in conventional stress-rupture racks. Specimens were wrapped in tantalum foil to inhibit contamination of the alloys by residual oxygen in the furnace atmosphere. Stress-rupture test results were extrapolated to a common time parameter hours) using the following experimentallydetermined parameter:

Log (100-hour rupture strength)=Log (applied stress)+0.16 Log (rupture time)0.32

This value of rupture strength, when compared with the expected value for rupture strength calculated from the previously defined relationship allowed analysis of the effects of auxiliary elements upon stress-rupture properties.

Materials for the examples were prepared and tested in accordance with the procedures set forth above. A tabulation of test results of the examples is illustrative of the achievements of the invention and is presented in Table 1. To achieve a clearer understanding of the attributes of the invention, the various examples illustrative of the invention are discussed individually below.

EXAMPLES A1 THROUGH A3 The following alloys, parts expressed as percentages by Weight, were prepared and evaluated:

TABLE 1.STRUCTURAL CONDITION AND TROPERTIES OF SOME COLUMBIUM ALLOYS Room-Temperature Tensile Properties 2200 F. Tensile 2,200 F. Stress-Rupture Behavior Properties Deviation of Esti- Exarnple 0.2% Offset Ultimate Reduc- Ultimate Rup Plastic Estimated Expected mated From Ex- Yield Tensile Elontion in Tensile Elon- S tress, ture Elon- 100-131 our 100H our pected 100-H our Strength, Strength, gation, Area, Strength, gation, K s.i. Time, gation, Rupture Rupture Rupture Strength K s.i K s.i. percent percent K s.i. percent hours percent Strfirigth, Stiength,

s.1. s.i.

K s.i. Percent l 79 97 22 50 31 21 15 41 44 13.0 13.1 --0.1 1 106 17 70 44 20 13 28 42 9. 4 13.3 -3. 9 29 112 19 49 42 4 18 11 46 12.7 13.6 -0.9 7 99 l 3 35 77 53 14. 4 14. 4 0 0 110 0 3 60 16 9 65 13.6 14. 0 1.0 7 108 12 10 38 49 15 50 12.0 12. 8 0.8 -6 125 2 0 50 17 10 75 11.8 13.1 -1.3 -10 3O 29 38 24. 6 24. (i 0 0 173 0 0 30 62 27. 8 27.2 +0. 6 +2 a0 74 40 2s. 6 2s. 8 -0. 2 -1 142 2 '10 59 14 30 1. 8 30 15. 8 20. 8 6. 0 29 141 3 12 54 15 27 27 22 21.9 20.8 +1.1 +5 139 3 14 64 22 27 3. 2 15. 2 21.3 0.1 29 147 2 9' 27 48 30 24. 0 21. 3 2. 7 +13 142 2 4 27 20 42 20.9 20.8 +0. 1 135 3 l0 30 23 35 23. 7 21. 3 +2. 4 -11 153 1 4 27 34 20 22. 7 20. 5 +2. 2 +11 1 VH=very high amount of retained cold work, reduced 90% at 1,800 F.; H=high amount of retained cold work, reduced at RT500 F.-

M =moderate amount of retained cold work, reduced 60% at 1,800 1 =low amount of retained cold work, reduced 60% at 29003100 F. 2 Temperature to initiate recrystallization in 1 hour. Example A] Columbiurn 85 Tungsten 15 (Cb-15W) Example A2 Columbium 80 Tungsten 15 Hafnium 5 (Cb-15W-5Hf) Example A3 Columbium 75 Tungsten 15 Hafnium 10 All'wcre fabricated toeffcct the structural condition H. In this constant structural condition, a degrading effectof hafnium upon recrystallization behavior is apparent; whereas recrystallization of the Cb-15W alloy (Example Al) was not initiated in 1 hour at 2200 F. (demonstrating some useful degree of structural stability at this temperature), the addition of 5 percent by weight of hafnium (Example A2) eliminated this degree of structural stabilityat 2200 F., and the 10 percent hafnium addition (Example A3) further degraded structural stability, as judged by the recrystallization temperatures cited in Table 1. As heretofore described, all samples for subsequent mechanical property evaluation were "stress-relief annealed for /2 hour at 2200 F. It is considered probable that this heat treatment did not result in measurable recrystallization of either the Ola-15W (Example A1) or the Cb15W-5Hf (Example A2) alloys but probably effected an appreciable degree of recrystallization in the Cb-ISW-IOHf alloy (Example A3).

Tensile tests at room temperature showed progressive strengthening attributable to the hafnium additions to Cb-15W. No significant effects of hafnium upon roomtempera-ture ductility were observed. At 2200 F., the 5 percent addition of hafnium to the Cb-15W alloy significantly increased its strength in the short-time tensile test without affecting ductility. However, at the 10 percent level, hafnium was no more effective as a short-time tensile strengthener than at the 5 percent level, and ductility was impaired.

In the time-dependent stress-rupture tests at 2200 F. the degradation in rupture strength effected by the 5 percent hafnium addition to the Cb-ISW alloy (Example A2) is immediately obvious. Whereasthe more stable Cb-15W (Example A1) alloy exhibited about the expectcd degree of rupture strength, the unstable Cb15W SHf (Example A2), when tested under time-temperature conditions that permitted the initiation of recrystallization, was far weaker than had been expected. The Cb- 15W-10Hf alloy (Example A3) in which recrystallization had been initiated prior to stress-rupture testing exhibited improved rupture strength compared to the Cb-15W-5Hf alloy (Example A2) but was still slightly below the usual limits (:5 percent) of expected rupture strength. Thus, it was established that in the heavily worked condition, hafnium additions to structural columbium alloys are definitely not beneficial to stress-rupture strength.

EXAMPLES B1 AND B2 The following alloys, parts expressed as weight, were prepared and evaluated:

percentages by Example B1 was fabricated to result in heavy work hardening, and Example B2 was fabricated to result in only moderate work hardening. The difference in the structural condition resulting from the fabrication historywas sufiicicnt to obscure the expected detrimental effects of hafnium upon resistance to recrystallization. The pronounced increase in tensile strength at room temperature and at 2200 F. resulting from the addition of 5 percent hafnium to the alloy base was accomplished without any significant effect upon ductility at either temperature. 7 r

Stress-rupture tests at 2200 F. again showed the hafnium-free alloy, Cb2OTa-15W (Example B1), to achieve the expected rupture strength. Despite indicated .good structural stability of the Cb20Tal5W5Hf alloy (EX-. ample B2) in the moderately worked condition, no benefit of hafnium upon rupture strength in this alloy was observed.

ll EXAMPLES 01 AND 02 Structural Cb-W alloys containing auxiliary alloying additions of vanadium and hafnium, parts expressed as percentages by weight, were prepared and evaluated:

Example C1 Columbium 82 Tungsten 15 Vanadium 3 (Cb15W-3V) Example C2 Columbium 77 Tungsten 15 Vanadium 3 Hafnium 5 Conditions of fabrication resulted in retention of moderate work hardening in each alloy. Comparison of the recrystallization temperatures for these alloys with the similar vanadium-free alloys of Examples A1 through A3, as cited in Table 1, show the detrimental effects of vanadium and hafnium upon recrystallization behavior, when differences in fabrication history are considered.

Examination of the room-temperature and 2200* F. tensile and 2200 F. stress-rupture data [for alloys of these examples and comparison of these data with those for the alloys of Examples A1 and A2 demonstrates:

(1) Both vanadium and hafnium exhibit substantial strengthening in short-time tensile tests.

(2) Eithervanadium, or the less severe condition of prior fabrication, or both, effect substantial degradation of tensile ductility at room temperature (comparing Example 01 with Example Al, and Example C2 with Example A2 in Table l). The admittedly substantial difference in room-temperature ductility between Example Cl and Example C2, rather than being directly attributable to the hafnium addition in Example C2, is probably a result of the increase in atomic concentration of tungsten and vanadium in Example C2 effected by the hafnium addition.

(3) Modest vanadium additions are slightly detrimental to the rupture strength of structural alloys. Reinforcing the data of Examples B1 and B2, the hafnium addition in Example C2 does not benefit stress-rupture strength of alloys fabricated using moderate fabrication procedures.

EXAMPLES D1 THROUGH D3 Alloys of the following compositions, parts expressed as percentages by weight, were fabricated and evaluated:

Example D1 Columbium 75 Tungsten 15 Molybdenum (Cb-W-10Mo) Example D2 Columbium 55 Tantalum -t 2O Tungsten 15 Molybdenum 1'0 (ObTa-15W-'10Mo) Example D3 Columbium 70 Tungsten 2O Molybdenum 10 These alloys were all fabricated to effect only light retained work hardening. Critical recrystallization temperatures were high, and structural stability of these alloys at EXAMPLES El THROUGH E4 The following alloys, parts expressed as percentages by weight, were prepared to effect work-hardening, as shown in the summary below, and evaluated:

Examples E1 and E2 Columbium 60 Tantalum 2'0 Tungsten 15 Molybdenum 5 (Cb20Ta-15W-5Mo) Examples E3 and E4 Columbium 55 Tantalum 20 Tungsten 15 Molybdenum 5 Hafnium 5 (Cb-20Ta-15W-5Mo-5Hf) Retained Example Work- Alloy H ardening Cb-20Ta-15W-5Mo Cb'20Ta15W-5M0 (lb-20'Ia-l5W-5Mo-5Hf E4 ob-20Ta-15W-5Mu-5Hr Both basic alloys in all conditions exhibited recrystallization temperatures that indicated at least fair structural stability at 2200 F. Room-temperature ductility was essentially unaffected by either the hafnium addition or the prior (fabrication condition. In the VH fabricated condition, the addition of hafnium was mildly strengthening in tensile testing at 2200 E, but, surprisingly and unaccountably, was mildly weakening at room temperature (an item of little significance to the protracted use of such alloys). Stress-rupture testing at 2200 F. showed this VH" condition of fabrication to be decidedly detrimental to rupture strength, suggesting structural instability despite indications to the contrary by virtue of the observed recrystallization temperatures. In this structural condition, hafnium additions are not particularly attractive.

The moderate condition of fabrication permitted achievement of about the expected value for rupture strength in Example E2. As shown by Examples D1 through D3, the light condition of fabrication would not significantly improve the rupture strength. However, in accordance with the new and unexpected beneficial result of this invention, a pronounced (13 percent) improvement in stress-rupture strength was achieved by Example E4, the alloy containing hafnium which was fabricated lightly. It is further of primary importance that this improvement was achieved with no concurrent sacrifice in room-temperature ductility.

EXAMPLES F1 AND F2 13 Example F2 Columbium 72.8 Tungsten 16.7 Molybdenum 5.5 Hafnium Example F1 was fabricated to produce a moderately work hardened structure, and Example F2 was fabricated to produce a lightly work-hardened structure. Both alloys exhibited excellent structural stability at 2200 F. as attested to by their recrystallization temperatures of 25005 and 2600 R, respectively. As in comparing Example B2 with Example B1, the detrimental effects of hafnium upon shadowed by the milder degree of fabrication of Example F2 compared with Example F1. The room temperature tensile strengths alloys of Example F were in general accord with expectations, based on alloy content and prior fabrication history. Examples F l and F2 displayed about equal room temperature ductility, despite their differences in fabrication history.

The moderately fabricated hafnium-free alloy, Example F1, exhibited the expected rupture strength at 2200 F. However, in good agreement with Examples E1 through E3 and the unexpected new and beneficial result of this invention, the lightly fabricated hafniumcontaining alloy, Example F2, demonstrated significant increase (11 percent) in 2200 F., IOO-hour stress-rupture strength. In accordance with the invention, this improvement was again achieved with no detrimental effect upon low-temperature ductility. I

EXAMPLE G An alloy of the following composition, parts expressed as percentages by weight, was fabricated to produce light work hardening, and evaluated:

Because of the combined additions of 15W, 5M0, and 3V, no measurable ductility at room temperature was expected. However, when tested, the alloy of this example did display some measurable room-temperature ductility, and at a strength level much greater than that of the moderately fabricated lower alloy-level material of Example C2, which exhibited about the same amount of ductility at room temperature.

The stress rupture strength at 2200" F. for this alloy was appreciably greater (11 percent) than expected, again demonstrating significant improvement in stress-rupture strength attributable to hafnium additions when the alloys are fabricated judiciously.

The foregoing examples, in accordance with the invention, demonstrate that hafnium is a very useful auxiliary strengthening addition to structural columbium-base alloys when treated by the method of this invention. Specifically, the following attributes of the invention are shown by the examples:

(1) Hafnium, when added to structural columbium-base alloys in quantities of up to percent by weight, increases the short-tirne tensile strength of these alloys without significant degradation of their low temperature ductility properties. The most beneficial results accrue when hafnium additions are within the range from 0.5 to 5 percent by weight.

(2) When fabricated under conditions to effect the retention of at least moderate degrees of retained work hardening, hafnium additions are not particularly desirable for improving the stress-rupture strength of structural alloys at temperatures of about 2200 F.

recrystallization behavior are overposed toanticipated service (3) When fabricated under conditions to effect light retained work hardening, alloys containing auxiliary hafnium additions of up to 10 percent by weight exhibit markedly improved stress-rupture strength up to at least about 2200 F.; this improved hot strength is achieved without sacrifice of low-temperature ductility.

Moreover, hafnium-free alloys fabricated under these conditions do not show the improvement in strength that is characteristic of hafnium-containing alloys that are otherwise similar.

(4) Fabrication at temperatures of from 2800" to 3200 F. impart to hafnium-containing structural columbiumbase, alloys unusually superior rupture strengths without any attendant degradation of low-temperature properties.

As mentioned earlier, hafnium, because of its reactive nature, in addition to normal solid-solution-strengthening effects, may achieve strengthening by virtue of its aflinity for interstitial elements, such as carbon, oxygen, and nitrogen, either as deliberate additions or unintentional or residual contaminantsof the alloys. Achievement of superior properties of judiciously fabricated hafnium-containing alloys as taught herein may result from either normal solid-solution strengthening, or from strengthening derived from interaction between hafnium and appropriate residual interstitial elements, or from a combination of these mechanisms. The most desirable fabrication conditions (temperatures of 28003200 F.) would be expected to enhance the potential for both strengthening mechanisms.

Deliberate additions of interstitial elements, particularly carbon, to columbium alloys containing reactivemetal (Zr, Hf, Ti) additions have frequently been employed to enhance the hot strength of columbium alloys. It has been found, however, that when such additions are made to alloys containing both substantial quantities of solid-solution-strengthener additions, such as tungsten and molybdenum, and lesser additions of reactive metals, fabricability and low-temperature ductility are seriously impaired. Moreover, published data show that such alloys are sensitive to structural changes by virtue of alteration of the second phase that is fornented, particularly by carbon additions, when the alloys are extemperatures. It is apparent that any such gross structural changes during high-temperature service would be highly undesirable. For these reasons, deliberate additions of interstitial elements are preferably excluded from the alloys that achieve the most desirable effects ,as taught by this invention, although residual interstitial elements (generally, maximums of weight of 0.01 percent carbon, 0.02 percent oxygen, and 0.01 percent nitrogen) are not harmful.

From the foregoing description ofthis invention, it is apparent that the key parameters in achieving the new and useful result of this invention are the amount and temperature of final reduction imposed upon a prior recrystallized metallic structure. As used in this application, a recrystallized metallic structure is defined as a strain-free structure, i.e., one that has had. all residual Work hardening removed from it, and in this connotation a recrystallized metallic structure includes the ascast structure of the alloys of this invention.

One of the important advantages of the invention is that its new and useful result is achieved by imposing a final reduction of the proper amount and temperature on a recrystallized or strain-free structure and that a strain-free state in the alloy specimen being processed may be introduced at any time during the processing by the use of a recrystallizing annealing treatment. The invention, however, provides great flexibility in the manner in which the final reduction parameters are imposed on the alloys. The important criteria are the total amount and temperature of final reduction imposed on the alloy after commencing from a strain-free or recrystallized base structure. In some instances, it may be preferable to impose the desired amount and temperature of final 15 reduction in one step. In other instances, it may be desirable to break down the imposition of the desired amount of final reduction into two, three, or more steps.

It is important to note that as defined in this application, reduction means reduction in cross-sectional area of the alloy specimen. The new and useful result of this invention can be achieved by use of any of the known methods of reduction employed in fabrication of alloys similar to the alloys of this invention. In practicing the invention it does not matter Whether the reduction in cross-section area is achieved by strip rolling, sheet rolling, tensile straining, forging, extrusion, wire drawing, or any other method of fabrication, so long as the desired amount of reduction in cross-sectional area is achieved.

The invention in its broader aspects is not limited to the specific details shown and described, but departures may be made from such details within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

. What is claimed is:

1. A method for improving the high-temperature stressrupture strength of a columbium-base alloy with-out sacrificing its low-temperature ductility properties and its structural stability during subsequent fabrication or service at high temperatures, the columbium-base alloy consisting essentially, by Weight, of:

Percent Hafnium up to 10 A solid solution strengthener selected from the group consisting of tungsten, molyband mixtures thereof 5 to 30 Tantalum Oto 40 Zirconium to5 Vanadium 0 to Beryllium 0 to 5 Essentially columbium balance said alloy containing not more than by weight of zirconium, vanadium, and beryllium, in aggregate; and the method comprising the steps of heating said alloy to a temperature of 2800 to 3200 F., and subjecting said alloy, from a strain-free state, to final reduction forces of a nature such that plastic deformation of the alloy is effected and the cross-sectional area of the alloy is reduced by an amount from 40 to 90% of the initial cross-sectional area.

2. The method of claim 1 in which the solid solution strengthener is selected from the group consisting of tungsten in an amount of to 25% by Weight of the alloy, and molybdenum in an amount of 10 to by weight of the alloy.

3. The invention as defined in claim 1, in which the alloy also includes tantalum in an amount of from 15 to 40% by weight of the alloy.

4. The invention as defined in claim 1, in which the alloy is heated to a temperature of from 2900" to 3100" F. and the force is of such a nature that the alloy is reduced in cross-sectional area by an amount of from 40 to 90% of the initial cross-sectional area.

5. The invention as defined in claim 1, in which the alloy is heated to a temperature of from 2900 to 3100 F. and the force is of such a nature that the alloy is reduced in cross-sectional area by an amount of from to of the initial cross-sectional area.

6. The invention as defined in claim 1, in which hafnium is present in an amount of from 0.5 to 5% by weight of the alloy.

7. The invention as nium is present in an amount of from 2 to 3% of the alloy.

8. The invention as defined in claim 1, in which the solid-solution-strengthener consists essentially of tungsten in an amount of from 10 to 20% by weight and molybdenum in an amount of from 2 to 10% by weight.

9. A columbium-base alloy having excellent hightemperature stress-rupture strength, room-temperature ductility, and structural stability during fabrication and service at 'high temperatures, the alloy consisting essentially,

defined in claim 1, in which hafby weight by weight, of: 7 Percent Hafnium up to 10 A solid solution strengthener selected from the group consisting of tungsten, molyband mixtures thereof 5 to 30 Tantalum 0 to 40 Zirconium 0 to 5 Vanadium 0 to 5 Beryllium 0 to 5 Essentially columbium balance said alloy containing not more than 10% by weight of zirconium, vanadium, and beryllium, in aggregate, and being characterized by having a light retained work-hardened structure imparted by reducing the cross-sectional area of a previously strain-free specimen of the alloy by from 40 to 90% while the alloy is at a temperature of from 2800 to 3200 F.

References Cited UNITED STATES PATENTS 3,034,934 5/1962 Redden l48-11.5 3,113,863 12/1963 Chang et al. --l74 3,193,385 7/1965 Jafie et al. 75-174 DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner. H. F, SAITO, Assistant Examiner. 

1. A METHOD FOR IMPROVING THE HIGH-TEMPERATURE STRESSRUPTURE STRENGTH OF A COLUMBIUM-BASE ALLOY WITHOUT SACRIFICING ITS LOW-TEMPERATURE DUCTILITY PROPERTIES AND ITS STRUCTURAL STABILITY DURING SUBSEQUENT FABRICATION OR SERVICE AT HIGH TEMPERATURES, THE COLUMBIUM-BASE ALLOY CONSISTING ESSENTIALLY, BY WEIGHT, OF: 